1. Field of the Invention
The present invention relates generally to an apparatus and method for transmitting/receiving signals in a communication system, and in particular, to an apparatus and method for transmitting/receiving uplink (UL) quasi-dedicated channel signals.
2. Description of the Related Art
The next generation communication system has developed into a packet service communication system. The packet service communication system, which is a system for transmitting burst packet data to a plurality of mobile stations (MSs), has been designed to be appropriate for high-capacity, high-speed data transmission. In addition, the next generation communication system is developing into an advanced system for guaranteeing mobility and Quality-of-Service (QoS) for a Broadband Wireless Access (BWA) communication system such as a wireless Local Area Network (LAN) communication system and a wireless Metropolitan Area Network (MAN) communication system, for high-capacity, high-speed data transmission, and the typical advanced communication system is an Institute of Electrical and Electronics Engineers (IEEE) 802.16e communication system.
The IEEE 802.16e communication system employs Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) scheme to support broadband transmission on physical channels of the wireless MAN communication system. With reference to the diagram FIG. 1, a description will now be made of a configuration of a general IEEE 802.16e communication system.
Referring to FIG. 1, the IEEE 802.16e communication system has a multi-cell topology, i.e., has a cell 100 and a cell 150. The IEEE 802.16e communication system includes a base station (BS) 110 for managing the cell 100, a BS 140 for managing the cell 150, and a plurality of MSs 111, 113, 130, 151 and 153. Signal exchange between the BSs 110 and 140 and the MSs 111, 113, 130, 151 and 153 is achieved by OFDM/OFDMA scheme.
A description will now be made of a UL resource allocation operation of a BS in a general IEEE 802.16e communication system.
A description will first be made of the possible cases where the BS should allocate UL resources.
In a first case, in order to feed back to the MSs to which the BS provides a service Channel Quality Information (CQI) indicating a channel state of each of the MSs, the BS should allocate UL resources for the CQI feedback. The IEEE 802.16e communication system defines a CQI feedback period as a period corresponding to a multiple of an OFDM frame. Therefore, for CQI feedback, the BS allocates CQI channels (CQICHs) to the MSs on a dedicated basis at periods of a multiple of the OFDM frame.
Generally, however, the CQI feedback period for the CQI feedback is determined taking into account the coherence time corresponding to moving velocities of the MSs. Because of a difference between the moving velocities of the MSs, the IEEE 802.16e communication system, which defines the CQI feedback period as the period of a multiple of the OFDM frame for all the MSs, causes a UL resource waste due to the CQICH allocation.
In a second case, when providing particular services to the MSs, the BS should allocate UL resources for providing the particular services. Herein, the term “particular service” refers to a service that necessarily requires UL resource allocation to enable data transmission over the UL in providing the service, such as Unsolicited Grant Service (UGS), realtime Polling Service (rtPS), and extended realtime Polling Service (ertPS).
The UGS represents the service that allocates UL dedicated resources, i.e., UL dedicated sub-channels, to an MS so that it can directly transmit data through the UL resources without sending a separate UL resource allocation request, i.e., without separately transmitting a Bandwidth Request (BW-REQ) message. Generally, the UGS is appropriate for Constant Bit Rate (CBR) traffic, for which a periodic fixed data rate should be guaranteed.
The rtPS represents the service that periodically allocates UL resources for a UL resource allocation request so that an MS may request the UL resource allocation, and is appropriate for periodic UL realtime service having a variable data rate, like a video streaming service. For convenience, the UL resource for the UL resource allocation request will be referred to as “UL resource allocation request-UL resource.”
The ertPS is a service appropriate for Voice over Internet Protocol (VoIP) traffic having silence suppression. The ertPS represents the service that requests UL resources in the form of piggyback in an ON interval and provides a service in the form of the rtPS in an OFF interval.
With reference to the signaling diagram of FIG. 2, a description will now be made of a UL resource allocation operation for providing the rtPS in a general IEEE 802.16e communication system.
Before a description of FIG. 2 is given, it should be noted that the rtPS, because of its characteristics, should periodically allocate UL resource allocation request-UL resources to an MS so that the MS may request UL resource allocation. In addition, the BS should allocate downlink (DL) resources using UL resource allocation information for the periodically allocated UL resource allocation request-UL resources, and poll the allocated DL resources to the MS.
Referring to FIG. 2, a BS 200, as it provides rtPS to an MS 250, allocates UL resource allocation request-UL resources to the MS 250 according to a preset polling interval, and polls UL resource allocation information for the allocated UL resource allocation request-UL resources to the MS 250. Herein, the polling interval is determined taking a delay bound of the rtPS into account. However, in the rtPS, even though its service characteristic has a variable data rate, not only the UL resource allocation request-UL resources are allocated according to the polling interval determined taking the delay bound into account, but also UL resource allocation information for the allocated UL resource allocation request-UL resources should be polled to the MS 250 through DL resources.
However, although the BS 200 allocates UL resource allocation request-UL resources according to the polling interval, the MS 250 transmits a BW-REQ message for UL resource allocation request on a burst basis at relatively long periods as shown in FIG. 2. Therefore, for the interval where the BW-REQ message is not transmitted, the UL resource allocation request-UL resources allocated to the MS 250 cannot be allocated to the other MSs being serviced by the BS 200, thus causing a waste of the UL resources and also wasting the DL resources used for polling the UL resource allocation information for the allocated UL resource allocation request-UL resources.
In particular, the IEEE 802.16e communication system transmits resource allocation information, i.e., DL resource allocation information and UL resource allocation information, to the MSs through a DL MAP message and a UL MAP message, respectively. However, because every MS should be able to receive the DL MAP message and the UL MAP message, the DL MAP message and the UL MAP message are, before being transmitted, modulated and coded according to the most robust Modulation and Coding Scheme (MCS) level among MCS levels available in the IEEE 802.16e communication system. Therefore, the transmission of the UL resource allocation information for the UL resource allocation request-UL resources to provide the rtPS causes a considerable waste of DL resources.
As described above, the current IEEE 802.16e communication system suffers a waste of UL resources due to CQICH allocation without consideration of mobility of the MSs and also suffers a waste of UL resources and DL resources for providing the rtPS. The waste of DL/UL resources results in deterioration of the entire system performance of the IEEE 802.16e communication system.